1. Field of the Invention
This invention relates to a bar for supporting a movable web as the web is transported and redirected from a first direction of movement to a second direction of movement, and in particular, to a turning bar which generates an air cushion to support, stabilize and remove wrinkles from the web as it makes the turn.
2. Description of the Prior Art
A variety of systems and devices are known in the prior art for developing an air cushion to support a moving web as the web changes direction. The purpose of such air cushion is to support the web free of engagement with a solid member during web transport to prevent damage and wear to the web.
Examples of such prior art devices are disclosed in Johnson U.S. Pat. No. 3,567,093, issued Mar. 2, 1971, Johnson U.S. Pat. No. 3,498,515, issued Mar. 3, 1970, Hamlin et al. U.S. Pat. No. 3,679,116, issued July 25, 1972, Sander U.S. Pat. No. 4,043,495, issued Aug. 23, 1977, Reba U.S. Pat. No. 4,136,808, issued Jan. 30, 1979, and Daane U.S. Pat. No. 4,197,972, issued Apr. 15, 1980.
Many of the prior art turning bar devices employ a plurality of apertures or slits in pressurized turning bar housings through which pressurized air passes and provides a direct support for the web during transport. It will be appreciated that devices of this nature use inordinately large amounts of pressurized air to accomplish the purpose of web support and are consequently inefficient and expensive to operate. In addition, such devices, due to uneven air distribution found at the turning bar surface, can cause wrinkling and other deformations to occur in the web. Prior art devices are also often characterized by even greater inefficiencies when operating at high speeds.
In the above-noted U.S. Pat. No. 4,197,972, issued to Daane, a turning bar device is illustrated which utilizes the Coanda effect to provide an air cushion for a web. When utilizing the Coanda effect ambient air is entrained to provide a portion of the web cushion and thus operating efficiencies are attained. The present invention also relates to a turning bar arrangement employing the Coanda effect to provide an air cushion. The present invention, however, employs a specific arrangement for bringing the Coanda effect into play, an arrangement that is specifically adapted to ensure even air distribution to the Coanda nozzle forming a component of the turning bar. Even air distribution is important to maintain proper web stability and control as the web makes the turn about the bar. In the present invention web control can be readily accomplished without the necessity of employing air nozzles disposed along the edges of web travel as is the case in Daane to eliminate side drift or lateral tracking instability of the web as well as to ensure predetermined desired spacing of the web with respect to the turning bar.
Another feature of the present invention resides in the employment in the turning bar of means to crown the web as it makes the turn, thus removing wrinkles therefrom and contributing to web stabilization. The invention is particularly useful with wet coated webs wherein contact could disrupt or cause defects in the coating.